I made a custom shape with the knife tool but I cant use the subsurf of it for sculpting
An n-gon is face with more than 4 sides. 3 sides = tri,  4 sides = quad, 5+ sides = n-gon. Although the use of tris and n-gons are sometimes a common part of modelling, both of them can be responsible for shading artifacts, especially near edges that are shaded smooth. More importantly to your scenario, is that blender calculates subdivisions best based on quad-only based geometry. See the example images below. The first images shows 2 planes - one with quad topology and the other with tris and n-gons. The second shows what happens to each of them (topology wise) when a subsurf modifier is added.


See the difference? This is one reason why people argue that you should keep your geometry as quad based as possible. A MULTI-RESOLUTION MODIFIER is a tool that aids in this pursuit. It is a modifier that can quickly reduce whatever geometry you have to being quad based (among other things) without loosing too much detail. Once the surface of your pommel is quad only, you can then subdivide and get the results you are looking for.